1. Field of the Invention
The present invention relates to a synthetic aperture radar system and a platform position measuring apparatus used in the same, and more particularly to a technique of increasing the measuring precision of a position of a flying body on which a platform is mounted.
2. Description of the Related Art
A synthetic aperture radar (SAR) is mounted on an artificial satellite and an aircraft and is used to obtain an image of a portion of a ground surface in a high resolution regardless of night and day and the weather of the ground surface portion.
FIG. 1 is a block diagram illustrating a basic structure example of a conventional synthetic aperture radar. Referring to FIG. 1, a synthetic aperture radar basic unit 10 and a position and attitude measuring apparatus 14 are mounted on a platform (not shown). In the synthetic aperture radar basic unit 10, a pulse signal is generated by a transmitter 11 and is radiated as a electromagnetic wave for the ground from a transmission and reception antenna 12. The electromagnetic wave is reflected on the ground surface and is received by the transmission and reception antenna 12. The received electromagnetic wave is amplified and detected by a receiver 13 and is recorded on a recording medium (not shown) such as a magnetic tape in a complex data format by a data recording unit 15.
A series of operations are repeated in a predetermined time interval of 1 msec. or at the frequency of 1000 Hz. Also, a position and attitude data of the platform measured by the position and attitude measuring unit 14 is also recorded by the data recording unit 15 together with the receive data by the synthetic aperture, radar basic unit 10.
After the measurement, an image is produced from the recorded data by an SAR (Synthetic Aperture Radar) image reproducing unit 16 through an SAR image reproducing process which is well known. The SAR image reproducing process is described in, for example, the fourth chapter of "Remote Sensing for Resource Investigation: practical use series 5 Synthetic Aperture Radar (SAR)" by Yoshirou Iguchi (published from Resource Observation and Analysis Centers on Mar. 31, 1992, pp153-198).
When the fluctuation of the platform in position is large in an aircraft, a fluctuation compensating process is executed using the platform position and attitude data synchronous with a pulse signal in the measurement in the case of the SAR image reproducing process. Thus, it is necessary to prevent the degradation of a resolution of the reproduced image and the warp of the image due to the fluctuation of the platform.
The fluctuation compensating process is a process in which the variation of a phase of the reception signal is compensated or corrected based on the actual fluctuation of the platform, supposing that the platform flies on an ideal straight route at a uniform velocity. This process is well known. This process is described in, for example, "III. DATA PROCESSING" of "Repeat-Pass Interferometry with Airborne Synthetic Aperture Radar" by A. L. Grayet.al (IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, Vol.31, No.1, JANUARY 1993, pp.180-191).
In order to obtain an SAR reproduction image without the degradation of the resolution through the fluctuation compensating process, it is necessary to compensate the platform position and attitude data for the fluctuation of the platform in the measurement in precision of 1/8 or below of the measurement wavelength. Because the synthetic aperture radar system at present uses the wavelength from several cm to about tens of cm, the required detection precision of the fluctuation of the platform is from several mm to several cm.
Also, in recent years, a method of detecting a minute diastrophism to the extent of the wavelength of the measurement electromagnetic wave or below by unit of a differential interferometry type synthetic aperture radar is developed. This method is described in, for example, "Mapping Small Elevation Changes Over Large Areas: Differential Radar Interferometry" by A. K. Gabrielet.al (Journal of Geophysical Research, Vol.94, No.B7, 1989, pp.9183-9191).
In this case, it is necessary to detect the fluctuation of the platform in the precision higher than that of the detection of the minute diastrophism. The fluctuation compensating process is executed based on the detecting result. Therefore, it is required that the fluctuation of the platform is detected in the precision of about several mm for detection of the minute diastrophisms of several cm such as earthquake, volcanism, landslide, and land subsidence. In this way, it is very important to obtain the fluctuation data of the platform in a high precision, when a data is obtained by the synthetic aperture radar system.
Conventionally, as the position and attitude measuring unit which is mounted on the platform together with the synthetic aperture radar is generally used a global positioning system (GPS), an inertial navigation system or a hybrid navigation system of a combination of them. The position measurement precision is about several m in the hybrid navigation system, and about several cm in kinematic GPS using a carrier wave. Therefore, the above systems are insufficient in precision for the fluctuation compensating process in the synthetic aperture radar and the diastrophism detection in the differential interferometry type synthetic aperture radar.
A method of compensating for the fluctuation in the synthetic aperture radar and a method of measuring a position by the radar are described in Japanese Laid Open Patent Application (JP-A-Heisei 6-160515) to solve the above problem.
Next, the method of measuring the position of the platform on which the synthetic aperture radar is mounted will be described. FIG. 2 is a diagram illustrating the conventional method of measuring the position of the platform using the radar.
Referring to FIG. 2, a reference numeral 1 denotes a radar platform, and a reference numeral 2 denotes a platform flight track. A reference numeral 3 denotes an measurement object area. Reference numerals 4, 5 and 6 denote first, second and third repeaters. A reference numeral 7 denotes a phase compensation basing point. Here, the first repeater 4, the second repeater 5 and the third repeater 6 are arranged in different positions.
FIG. 3 is a block diagram illustrating the structure of a conventional fluctuation compensating and position measuring system in a synthetic aperture radar system. Referring to FIG. 3, the system is composed of a synthetic aperture radar basic unit 50, which is equivalent to the synthetic aperture radar basic unit 10 shown in FIG. 1. In the synthetic aperture radar basic unit 50 is composed of a transmitter 51, a transmission and reception switching unit 52, a transmission and reception antenna 53, a receiver 54 and a local oscillator 55. The synthetic aperture radar system is further composed of a phase compensation reference signal generating unit 56, a complex data multiplier 57, an image reproducing unit 58 and a display unit 59. Also, the synthetic aperture radar system is further composed of an inertial navigation system 60 and an antenna directional control unit 61. Moreover, the synthetic aperture radar system is further composed of antennas 70, transceivers 71, relative distance calculating units 72 and a platform position calculating unit 73. The antenna sends and receives a electromagnetic wave to and from a repeater. The transceiver 71 is connected with the antenna 70. The relative distance calculating unit 72 is connected with the transceiver 71 and the inertial navigation system 60 and calculates a relative distance between the radar platform 1 and the phase compensation reference point 7. The platform position calculating unit 73 determines the position of the platform from the relative distances calculated by the relative distance calculating units 72.
The operation of the above-mentioned system will be described with refers to FIG. 2 and FIG. 3. The operation of the synthetic aperture radar basic unit 50 is the same as described above. After the repeater receives and amplifies a high frequency signal which has been radiated from the radar, the repeater sends back the amplified high frequency signal to the received direction.
In the method of measuring the position by the radar, the high frequency signals are transmitted to the first repeater 4, the second repeater 5 and the third repeater 6 using the transmission and reception antennas 70 and the transceivers 71. The amplified high frequency signals are received from the first repeater 4, the second repeater 5 and the third repeater 6 by use of the transmission and reception antennas 70 and the transceivers 71.
In this case, a relative distance between the radar platform and the repeater is calculated from the reception signal which has been received by the above-mentioned transceiver 71 based on a electromagnetic wave propagation time and a phase by the relative distance calculating unit 72. The position coordinate of the radar platform is calculated from the above-mentioned relative distances and the positions of the repeaters by the platform position calculating unit 73 connected with the above-mentioned relative distance calculating unit 72.
In the above mentioned conventional method of measuring a position of the platform, the radar used for the position measurement is different from the radar used for the SAR. Therefore, 3 or more antennas and transceivers are required to send and receive the high frequency signal to and from the repeater, in addition to the synthetic aperture radar basic unit.
Also, the antenna to send and receive the high frequency signal to and from the repeater must be controlled to always face the repeaters. Therefore, a control mechanism become necessary. Thus, there is a problem that the structure of the synthetic aperture radar system becomes very complicated.
Also, the antennas which send and receive high frequency signals to and from the repeaters are different from each other. Also, the positions of the repeaters are different from each other. Therefore, in order to calculate the precise position of the platform, their relative position relation must be considered. Thus, there is a problem that the calculation of the platform position becomes very complicated.
In conjunction with the above description, an interferometry type synthetic aperture radar fluctuation compensating apparatus is disclosed in Japanese Patent No. 2546175. In this reference, the apparatus is composed of a an integration time calculating section, a summation section, a fluctuation compensating data calculating section and a fluctuation compensating section. The integration time calculating section calculates an integration time of an SAR reproduction process based on a position and speed data of a flying body. The summation section sums fluctuation data outputted from a recording and reproducing section based on the integration time. The fluctuation compensating data calculating section calculates a fluctuation compensating data based on the summation data and the position and speed data. The fluctuation compensating section calculates a difference between an interference data outputted from an interference processing section and the fluctuation compensating data and outputs the difference as a compensation interference data.
Also, an interferometry type synthetic aperture radar apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-230039). In this reference, the apparatus is composed of a radar section, a plural image generating section, an image mixing section and a phase unlapping section. The radar section performs measurement using a synthetic aperture radar apparatus to obtain a plurality of geometry images in which measurement conditions are different from each other. The plural image generating section combines the plurality of geometry images to interference to each other and generate a plurality of interference images. The image mixing section mixes the plurality of interference images. The phase unlapping section converts the mixed plurality of interference images into images with data corresponding to the geometrical height.
Also, a radar apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-29528). In this reference, the apparatus is composed of a signal processing section which performs a compensating process in a distance direction and a synthetic aperture process to a reflected signal from a target to generate a high resolution radar image signal. The signal processing section is composed of a reference point detecting section 121, a smoothing section 122, and a range walk compensating section 123. The reference point detecting section 121 detects a reference point of the target. The smoothing section 122 acquires the frequency of the reference point plural times from a plurality of transmission signals and specifies the frequency of the reference point from the plurality of frequencies to determine a speed. The range walk compensating section 123 synthesizes the speed of the target and the speed of a moving body externally given to determine the change of a relative distance to the target and compensates the position of a reflected signal from the target for every range cell corresponding to a resolution in a distance direction.
Also, a satellite mounted synthetic aperture radar apparatus is disclosed in Japanese Patent No. 2751901. In this reference, the apparatus is composed of an antenna section and a chirp pulse generator. The antenna section is mounted on a plane including a traveling direction of a satellite and radiates two beams for the ground and reflection beam of the radiated beams. The radiated beams have slightly different radiation angles in a perpendicular to the traveling direction. The chirp pulse generator generates chirp pulses corresponding to the two radiated beams such that the delay inclination characteristic of the chirp pulses are inverted for every pulse.
Also, an SAR/GPS inertial distance measuring method is disclosed in Japanese Patent No. 2702076. In this reference, an air plane is moved along a predetermined route, and a relative position and speed along the route is precisely measured using the GPS with an inertial navigation system. A first synthetic aperture radar map is generated and a target pixel is specified in the first synthetic aperture radar map corresponding to a target. Also, additional synthetic aperture radar maps are generated and a target pixel is specified in the additional synthetic aperture radar map. A position of the target pixel is calculated in the inertial navigation system and the GPS and at the same time a propagation speed of a radar wave to the target is calculated using the air plane position data calculated by the GPS. A precise position of the target pixel is calculated using the calculated radar wave propagation speed.